Knowledge of interactions between freshwater planktivores and their prey is critical for understanding the structure and dynamics of plankton communities and fish populations. The level of our understanding determines our ability to assess potential effects on lake ecology of changing conditions, such as climate change and eutrophication. We investigated the diel vertical distribution patterns of planktivores and plankton, and planktivore stomach contents in Lake Mendota and Trout Lake, WI, USA. These data, combined with published values, were used in bioenergetics models of both planktivores and plankton to examine the seasonal changes in predator/prey interactions. Results indicate that fish are more often restricted by temperature and oxygen conditions than by food vertical distribution, while plankton appear to respond more directly to predation risk from planktivores. These findings have important implications for studies of aquatic systems and for modeling strategies.